The Recent Uptime indication of a Validator shows the percentage of “proposals” the validator has successfully made over the past 500 epochs (roughly 2 weeks).
The record of completed proposals and missed proposals is saved on the Radix ledger - but only for the Top 100 validators. Since there is no record of completed or missed proposals for validator nodes outside the Top 100, it is not possible to show a Recent Uptime indication for these nodes.
Further reading:
https://learn.radixdlt.com/article/how-to-use-the-radix-explorer